The economic impact of infectious diseases in the poultry industry is well-appreciated. Immunization of birds has helped reduce the cost of production by decreasing the incidence of gastrointestinal, respiratory and systemic diseases. While vaccines provide adequate immunity for those pathogens against which a flock has been immunized, there are few vaccines which can provide broad-based cross-protection against unanticipated diseases or against those diseases for which an animal has not been specifically vaccinated.
A number of important diseases of domestic poultry are caused by bacteria able to invade host tissues, such as Salmonella spp., Escherichia spp. and Pasteurella spp. While many vaccines are available for immunization against individual species and serotypes, none provide cross-protection or stimulate broad-based immunity against multiple serotypes, species or genera.
One essential factor required for a bacteria to induce clinical disease is the ability to proliferate successfully in a host tissue. Iron is an essential nutrient for the growth of gram-negative bacteria in vivo, but is virtually unavailable in mammalian and/or avian tissues because the iron is either intracellular or extracellular, complexed with high affinity, iron-binding proteins, for example, transferring in blood and lymph fluids and lactoferrin in external secretions. In normal tissues, the concentration of iron is approximately 10.sup.-18 M, far below that required for bacterial growth.
To circumvent these restrictive conditions, pathogenic bacteria have evolved high affinity iron transport systems produced under low iron conditions, which consist of specific ferric iron chelaters, "siderophores," and iron-regulated outer membrane proteins (IROMPs) and/or siderophore receptor proteins (SRPs) which are receptors for siderophores on the outer membrane of the bacterial cell. Siderophores are synthesized by and secreted from the cells of gram-negative bacteria under conditions of low iron. Siderophores are low molecular weight proteins ranging in molecular mass from about 500 to about 1000 MW, which chelate ferric iron and then bind to IROMPs in the outer bacterial membrane which, in turn, transport the iron into the bacterial cell. Although the use of IROMPs as immunogens has been considered, these proteins have not been examined for such use, at least in part, due to an inability to extract these proteins from bacterial membranes in high volume and with a desired level of purity and immunogenic quality.
Accordingly, an object of the invention is to provide a method for obtaining high amounts of immunogenic quality siderophore receptor proteins from Escherichia coli, Salmonella, Pasteurella, and other gram-negative bacteria. Another object is to provide a vaccine for immunizing poultry and other animals against these bacteria. Yet another object is to provide a vaccine for cross-protection against multiple serotypes, species and/or genera of bacteria belonging to the family Enterobacteriaceae and/or Pasteurellaceae. A further object is to provide a diagnostic assay to monitor and/or profile sepsis and subclinical disease caused by gram-negative bacteria under field conditions.